Process for skid-proofing concrete surfaces



United States Patent C) 3,318,207 PROCESS FOR SKID-PROOFING CONCRETE SURFACES Jack Holton Whiteside, Plaquemine, Iberville Parish, La., assignor to Slip Pruf Service Corporation, Baton Rouge, La., a corporation of Louisiana No Drawing. Filed June 22, 1964, Ser. No. 377,062 8 Claims. (CI. 94-22) This invention relates to a process for treating surfaces which become slippery when wet with water. More particularly this invention relates to a process whereby the slip or skid resistance of wet concrete surfaces is increased and it is a companion of that in copending application Ser. No. 377,078 filed June 22, 1964.

As improvements have been continually made in providing concrete surfaces for vehicular and pedestrian traffic, the tendency has been to provide smoother surfaces than heretofore which, while improving ride, walking, and other travel characteristics, has increased the hazards created by using such surfaces when they are Wet. In concrete roadway surfaces, both in highway and driveway applications, such smooth surfaces in combination with higher travel speeds, have magnified problems associated with skid resistance and, in most instances, these cannot be overcome by improved tire-tread design and manufacture. It is obvious that these conditions pertain at intersections, curves and steep grades on highway surfaces, but more surprisingly, this condition has even been observed in driveways. In many public buildings, for purposes of architectural beauty and convenience of access, elevated or depressed roadways or driveways have become commonplace. Frequently such elevated or depressed driveways are constructed with substantial grade angles which pose no problem to vehicular trafiic from the standpoint of power of the vehicle, but create very definite skid problems or loss of traction in starting up, when the smooth concrete surfaces employed become wet. It is a frequent observation that elevated or depressed ramps to a newly constructed public building become unuseable by heavy trucks delivering supplies when such roadways are Wet.

Walkways, particularly around public buildings, and sidewalks are increasingly being produced of concrete material having, from other standpoints, desirable smooth characteristics. Such surfaces, while decorative, create distinct hazards when wet. Particularly hazardous situations arise from steel trowelled surfaces, ground and polished terrazzo and the like. Flooring in buildings frequently become wet, for example from pedestrian traffic entering from rain-soaked or snow-covered sidewalks, or in shower or locker rooms.

Several methods have been suggested for increasing the skid resistance or the traction on such surfaces, with more or less success. Physical treatments comprise incorporating abrasive or granular material, such as gravel, shale, broken stone, and the like, in the upper surface, or providing grooved or corrugated surfaces. Indeed, exposed granular material only increases the dry skid resistance, as such material itself usually becomes slippery when wet. Certain chemical treatment have been suggested, such as leaching away a portion of the surface with hydrochloric acid. However, none of these treatments is completely satisfactory, and indeed create secondary objections. For example, the decorative quality is destroyed, the desirable smooth riding or walking qualities are impaired and, furthermore, the results obtained are not compensated by the change in physical nature of the surface. Other considerations such as economy, hazards of treatment and lack of permanence are also drawbacks to these prior attempts to improve the traction or skid resistance. Furthermore, in many cases such treatment increases the porosity of the surface so that secondary disadvantages occur. For example, any treatment which substantially increases the porosity of a concrete surface also increases the absorption thereby of oils inevitably deposited thereon when in vehicle traific use, which actually increases the slipperiness of the surface when either wet or dry.

For the purpose of this description the term concrete can include material containing Portland or hydraulic cement, comprising lime, alumina, silica, and ferric oxide; low heat cement having higher tetracalcium aluminoferrate and dicalcium silicate content and less of tricalcium silicate and aluminate; H.E.S. cement made from raw materials having a high lime-to-silica ratio; aluminous cement, having thirty to thirty-five percent alumina; mortar; grout, and the like. In general, concrete as employed herein comprises a conglomerate of pebbles, gravel, broken stone, sand, blast-furnace slag or cinders as the aggregate, embedded in a matrix of mortar or cement. Also the term is intended to include cellular and light aggregate concrete. By extension it is also understood that this process is useful in treating asphaltic surfaces wherein an aggregate, as above, is exposed in the wearing surface.

It is, therefore, an object of this invention to provide a process for treating concrete surfaces to increase the slip resistance thereof when such surfaces are wet with water or oily liquids. In particular, it is an object of this invention to provide a process for treating such surfaces without at the same time changing substantially the visual appearance of the surface. It is a further object of this invention to provide concrete, mortar, shale, flint, terrazzo and the like surfaces which are resistant to slipping when wet, which retain this property in normal use, and which may be repeatedly cleaned without destroying this property, at the same time leaving the initial appearance of the surface unchanged either wet or dry,

The above and other objects of this invention are accomplished by applying under carefully controlled conditions to the defined surfaces, an aqueous solution of a water-soluble acid fluoride, having between about 0.1 and parts of fluoride ion per 1000 parts of water, leaving the solution in contact with the surface thereof for periods of time ranging from three seconds to one hundred twenty minutes, depending upon the concentration of the solution and the nature and condition of the surface being treated. It is a feature of this invention that the surface to be treated need not be cleaned and free from oils, greases, detergent films and the like prior to treatment, but in certain instances improved results are obtained, thereby. The method of application is not critical and can be by spraying, brushing, pouring or sponging the surface, so long as a uniform application is obtained and application time and concentration is controlled to achieve the desired increase in slip resistance without substantially changing the appearance.

Water-soluble inorganic acid fluorides are in general suitable for the process of this invention. One of the preferred solutions for treating concrete surfaces comprises aqueous hydrofluoric acid having between about 0.01 and 10 weight percent acid expressed as fluoride ion. Likewise, the alkali bifluorides, such as sodium bifluoride, potassium bifluoride, lithium bifluoride and ammonium bifluoride, are satisfactory. Suitable solutions of such bifluorides can be prepared by adding equivalent amounts of the alkali or ammonium fluoride to a hydrofluoric acid solution, thus, in effect forming solutions of the bifluorides in situ. Where such materials are available as articles of commerce, they can, of course, be directly dissolved in water in the concentrations indicated to form directly solutions for application tothe surfaces to be treated. Similarly, boric acid can be dissolved in hydrofluoric acid of the required concentration to result in solutions of fluoroboric acid having thel required activity and somewhat reduced volatility.

The principal advantages of employing such watersoluble fluoride salts resides in their low volatility and reduction of concurrent attack on portions of the article to be treated other than the treatment zone, that is in the vapor space above the solution. Hydrofluoric acid is, however, equally satisfactory, particularly in the lower concentration range, and can be successfully employed by taking normal precautions against vapor contact. If such vapor contact is not significant, or if conditions exist where adequate ventilation can be supplied, hydrofluoric acid is entirely satisfactory over the entire concentration range.

The following illustrates one method of determining the proper conditions of treatment to achieve the results of this invention. A solution of hydrofluoric acid of known concentration within the limits specified above is wiped as a thin film on a number of test pieces of smooth concrete block. At selected intervals of time, within the limits specified above, a solution of sodium carbonate is poured on each test piece followed by washing with water and drying. It is observed that those test pieces upon which the hydrofluoric acid had remained above a certain period of time will have a roughened appearance and a loss of the original smoothness, and an increased porosity. By this means is determined the maximum period of time that a solution of this concentration should remain on the surface to be treated, that is less than that required to produce a notable change. The remaining test pieces, that is those treated for less than this maximum period of time, are then wetted with water, and by applying pressure in a diagonally downward direction with the foot in a leather-soled shoe it will be determined that the test pieces treated for longer than a certain minimum period of time have a greatly increased resistance to slip over a similarly wetted, untreated test piece. This procedure is then repeated with other concentrations of hydrofluoric acid, or with the other acid fluoride solutions of this invention. From this can be determined the optimum concentration with each type of fluoride ion and the optimum time of treatment, for any such surfacing coming within the scope of this invention.

While the nature of the treatment is not well understood, it would appear that a subtle change in the surface occurs which is not merely an etching of discrete particles of the surface. This becomes evident when it is recognized that a hard-trowelled concrete floor surface can be rendered completely safe, having a high degree of resistance to slipping to the bare foot when wet, without it being in any way apparent by visual observation that the surface has been treated. The orginal texture imparted during construction is retained and the feel of the surface when dry is substantially unchanged. It is only when a film of water or oil is in contact with the surface that the effect of the process of this invention is noted.

The principal chemical method heretofore employed to improve the skid resistance of concrete surfaces has been the use of hydrochloric acid. However, the attack on concrete of hydrochloric acid is apparently by a different mechanism than that which occurs when employing solutions of this invention. Hydrochloric acid only roughens the surface and such a surface has the same wet slip characteristics as before. To be effective, hydrochloric acid must be used in high concentrations on the order of twenty percent or higher. In contrast, solutions of this process are effective in much higher dilution, as little as 0.1 percent being effective if maintained in contact with the surface for a sufficient length of time. Furthermore, the action of hydrochloric acid is to react immediately with the concrete and probably selectively with the cementitious rather than aggregate phase. As a result, it is difficult to spread hydrochloric acid effectively to provide uniform treatment and much of the hydrochloric acid is wasted as it is adsorbed vertically through the surface. In contrast, when applying acid fluorides of this invention the action is slow, moderate and easily controllable. Therefore, it is possible to spread a uniform layer of the acid fluoride solution on the surface being treated, resulting in substantially uniform improvement over the entire surface. It appears that the solutions of this invention attack the aggregate materials which, of course, are inert to hydrochloric acid. Thus a different effect is achieved. In any concrete surface containing an aggregate, the actual wear surface is the aggregate, as the softer cementations phase is rapidly worn or leached away. This is even true of steeltrowelled concrete. Therefore, the treatment of this invention achieves its effectiveness by increasing the skid resistance of the exposed aggregate phase, in contrast to the use of hydrochloric acid which further leaches away the cementitious phase. This leaching increases the exposure of the surface layer of aggregates, and when these aggregates become wet they retain their wet slip characteristics and no real improvement in slip resistance when wet has been achieved.

For external applications it is not critical to remove the treatment solution. The times indicated are, in such instances, the time to exhaustion of the active ingredient. However, for finer control, as with interior or decorative surfaces, the solution can be removed by washing with Water, by flooding or wiping, or neutralized as by soda ash, sodium bicarbonate, lime, ammonia and the like.

One method for determining the coeflicient of friction or increase in skid resistance for concrete surfaces following treatment by this process is by use of the Road Skid Resistance Tester developed by the Road Research Laboratory of the Department of Scientific and Industrial Research at Har-mondsworth, Middlesex, England. This tester comprises a framework for suspending a free-swinging pendulum, having attached to the lower end thereof a rubber slider, hinged and spring-loaded, so that as the pendulum describes a downwardly descending arc, passing over and in contact with a flat surface, the rubber slider remains in contact with the surface, describing a horizontal, planar movement. Rigidly attached to the pivotal end of the pendulum arm is a pointer moving across the face of a graduated scale. Thus, as the slider passes over and in contact with the test surface, the pointer describes a corresponding arc in circular travel from the point of contact to the position where the slider is stopped by friction. The percentage movement of the pointer on a test piece, as a function of the movement on a standard piece, is a measure of the relative skid resistance of the surface being tested. Thus, by adjustment of the pressure of the rubber slider and the arc of free-fall prior to contacting the surface, the machine can be adjusted to read percent of full scale on a flat, untreated road surface wetted with water. As a test of the effectiveness of the process of this invention, placement of the tester on a treated concrete road surface, similarly wetted with water and with the same machine adjustments, the pointer may describe, for example, seventy-two percent of the full scale are after treatment with a five percent solution of hydrofluoric acid. Upon observing these treated areas and the untreated area when dry the surfaces can not be distinguished from one another, that is the treated pieces and the untreated area have substantially the same resistance to skid when tested dry.

The following replicate data were determined by means of the Skid Resistance Tester at a heavily travelled downtown intersection in a city of a quarter of a million people. These data were obtained before and after applying a solution of five parts hydrofluoric acid per 100 parts of water from a spray applicator, allowing the solution to remain in contact with the roadway for twenty minutes and thereafter sluicing with water. The concrete road surface had been in use approximately ten years, and from constant wear, the aggregate was prominent on the surface.

READINGS WITH SKID RESISTANCE TESTER When this treatment is applied to a newly poured, smooth concrete road surface, an improvement of two to three times the above can be achieved.

From the above data it is apparent that on the dry, relatively clean portions of the intersection, a moderate change is observed in the dry surface and a significant change in the presence of oil slicks. Such an improve ment in terms of highway safety is a very significant advance in the art.

The following table illustrates concentrations of the indicated acid fluoride solutions which, when applied to a fresh smooth roadway surface for the period of time indicated, reduce the slipping characteristic of the surface when Wet by at least thirty-five percent.

Similarly, terrazzo can be rendered tenfold more resistant to slipping when Wet by employing the following materials at the indicated concentrations and time of application.

Concentration, Time of Material Weight Percent Treatment,

Minutes Hydrofluoric Acid 0. 01 110 D 0. 5 35 18. 0 0. 1 Ammonium Bifiuo 5. 0 1S Fiuoroboric Acid 10.0 8

The foregoing description of the process of this invention is illustrative of several means of accmplishing the objects thereof. However, it is not to be construed as limiting to the scope thereof, as other embodiments. and variations will be apparent to those skilled in the art. Thus, where the acid fluoride is available in powder form, this can be dusted on the surface followed by a water spray, resulting in solutions corresponding to the process of this invention. The scope and limits of this invention are to be construed, therefore, in the light of the appended claims.

I claim:

1. A process for treating a concrete surface to improve its resistance to skidding, which process comprises applying to such a surface an aqueous acid fluoride in a concentration equivalent to from 0.01 to 18.0 weight percent hydro-gen fluoride, to cause the acid fluoride to attack the concrete surface and reduce its slipperiness, and terminating the attack before the visual appearance of the surface has significantly changed.

2. The combination of claim 1 in which the concrete surface is the surface of a roadway.

3. The combination of claim 2 in which the aqueous acid fluoride is applied by pouring over the concrete surface.

4. The combination. of claim 1 in which the acid fluoride is hydrogen fluoride.

5. The combination of claim 1 in which the concrete surface treated is a hard-trowled surface.

6. A process for treating a roadway surface that has exposed aggregate to diminish the slipperiness of that surface, which process comprises applying to such a surface an aqueous acid fluoride in a concentration equivalent to from 0.01 to 18.0 weight percent hydrogen fluoride, to cause the acid fluoride to attack the surface and reduce its slipperiness, and terminating the attack before the visual appearance of the surface has significantly changed.

7. The combination of claim 6 in which the roadway surface is an asphaltic surface.

8. The combination of claim 2 in which the roadway is an outdoor roadway, the acid fluoride is about a 5% aqueous solution of hydrogen fluoride, and the treatment is terminated by the exhaustion of the hydrogen fluoride in the applied solution.

References Cited by the Examiner UNITED STATES PATENTS 2,652,360 9/1953 Bond et al 156-2 2,937,580 5/ 1960 Spalding ct -al 94-22 3,075,854 1/ 1963 Blubagh 9422 3,161,114 12/ 1964 Wittenwyler 94-22 FOREIGN PATENTS 164,100 6/ 1954 Australia.

ALEXANDER WYMAN, Primary Examiner. JACOB STEINBERG, Examiner. 

1. A PROCESS FOR TREATING A CONCRETE SURFACE TO IMPROVE IS RESISTANCE TO SKIDDING, WHICH PROCESS COMPRISES APPLYING TO SUCH A SURFACE AN AQUEOUS ACID FLUORIDE IN A CONCENTRATION EQUIVALENT TO FROM 0.01 TO 18.0 WEIGHT PERCENT HYDROGEN FLUORIDE, TO CAUSE THE ACID FLUORIDE TO ATTACK THE CONCRETE SURFACE AND REDUCE ITS SLIPPERINESS, AND TERMINATING THE ATTACK BEFORE THE VISUAL APPEARANCE OF THE SURFACE HAS SIGNIFICANTLY CHANGED. 